It is well known that nickel based superalloys are extensively employed in high performance environments. Such alloys have been used extensively in jet engines and in gas turbines where they must retain high strength and other desirable physical properties at elevated temperatures of a 1000.degree. F. or more.
Many of the nickel-based superalloys depend for part of their strength and other properties at high temperature on .gamma.' precipitates. Some detailed characteristics of the phase chemistry of .gamma.' are given in "Phase Chemistries in Precipitation-Strengthening Superalloy" by E. L. Hall, Y. M. Kouh and K. M. Chang, "Proceedings of 41st Annual Meeting of Electron Microscopy Society of America", August 1983 (p. 248).
The following U.S. patents disclose various nickel-base alloy compositions: U.S. Pat. Nos. 2,570,193; 2,621,122; 3,046,108; 3,061,426; 3,151,981; 3,166,412; 3,322,534; 3,343,950; 3,575,734; 3,576,681, 4,207,098 and 4,336,312. The aforementioned patents are representative of the many alloying situations reported to date in which many of the same elements are combined to achieve distinctly different functional relationships between the elements such that phases providing the alloy system with different physical and mechanical characteristics are formed. Nevertheless, despite the large amount of data available concerning the nickel-base alloys, it is still not possible for workers in the art to predict with any degree of accuracy the physical and mechanical properties that will be displayed by certain concentrations of known elements used in combination to form such alloys even though such combination may fall within broad, generalized teachings in the art, particularly when the alloys are processed using heat treatments different from those previously employed.
Most of the alloys that are processed either by cast and wrought or by powder metallurgy technologies can only retain their superior strength at temperatures up to about 1200.degree. F. A dramatic drop in strength is observed when the temperature of these alloys is raised above 1200.degree. F.
The objectives for nickel-base superalloys of this invention are to develop new alloy compositions which have high strength at temperatures above 1200.degree. F. and which have minimum time dependence of fatigue cracking resistance, and extended stress rupture life.
Another object for nickel-base superalloys of this invention is the provision of an alloy system which is not sensitive to different cooling treatments but which may be subjected to a range of cooling treatments without significant deterioration or loss of desirable alloy properties and particularly high strength at high temperature.
A problem which has been recognized to a greater and greater degree with many such nickel based superalloys is that they are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually initiate or propagate or grow while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to part fracture or other failure. The consequence of the failure of the moving mechanical part due to crack formation and propagation is well understood. In jet engines it can be particularly hazardous and can be catastrophic.
However, what has been poorly understood until recent studies were conducted was that the formation and the propagation of cracks in structures formed of superalloys is not a monolithic phenomena in which all cracks are formed and propagated by the same mechanism and at the same rate and according to the same criteria. By contrast the complexity of the crack generation and propagation and of the crack phenomena generally and the independence of such propagation with the manner in which stress is applied is a subject on which important new information has been gathered in recent years. The period during which stress is applied to a member to develop or propagate a crack, the intensity of the stress applied, the rate of application and of removal of stress to and from the member and the schedule of this application was not well understood in the industry until a study was conducted under contract to the National Aeronautics and Space Administration. This study is reported to a technical report identified as NASA CR-165123 issued from the National Aeronautics and Space Administration in August 1980, identified as "Evaluation of the Cyclic Behavior of Aircraft Turbine Disk Alloys" Part II, Final Report, by B. A. Cowles, J. R. Warren and F. K. Hauke, and prepared for the National Aeronautics and Space Administration, NASA Lewis Research Center, Contract NAS3-21379.
A principal unique finding of the NASA sponsored study was that the rate of propagation based on fatigue phenomena or in other words the rate of fatigue crack propagation (FCP) was not uniform for all stresses applied nor to all manners of applications of stress. More importantly, the finding was that fatigue crack propagation actually varied with the frequency of the application of stress to the member where the stress was applied in a manner to enlarge the crack. More surprisingly still, was the finding from the NASA sponsored study that the application of stress of lower frequencies rather than at the higher frequencies previously employed in studies, actually increased the rate of crack propagation. In other words the NASA study revealed that there was a time dependence in fatigue crack propagation. Further the time dependence of fatigue crack propagation was found to depend not on frequency alone but on the time during which the member was held under stress for a so-called hold-time.
Following the discovery of this unusual and unexpected phenomena of increased fatigue crack propagation at lower stress frequencies there was some belief in the industry that this newly discovered phenomena represented an ultimate limitation on the ability of the nickel based superalloys to be employed in the stress bearing parts of the turbines and aircraft engines and that all design effort had to be made to design around this problem.
However, it has been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates.
The development of the superalloy compositions and methods of their processing of this invention focuses on the fatigue property and addresses in particular the time dependence of crack growth.
Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (.sigma.) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity K, which is proportional to .sigma..sqroot.a. Under the fatigue condition, the stress intensity in a fatigue cycle represents the maximum variation of cyclic stress intensity (.DELTA.K), i.e., the difference between K.sub.max and K.sub.min. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (.DELTA.K) until the static fracture toughness K.sub.IC is reached. Crack growth rate is expressed mathematically as da/dN .varies.(.DELTA.K).sup.n. N represents the number of cycles and n is a constant which is between 2 and 4.
At high temperatures, however, the cyclic frequency and the shape of the waveform become the important parameters determining the crack growth rate. For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys.
The most undesirable time-dependent crack-growth behavior has been found to occur when a hold time is superimposed on a sine wave variation in stress. In such case a test sample may be subjected to stress in a sine wave pattern but when the sample is at maximum stress the stress is held constant for a hold time. When the hold time is completed the sine wave application of stress is resumed. According to this hold time pattern the stress is held for a designated hold time each time the stress reaches a maximum in following the normal sine curve. This hold time pattern of application of stress is a separate criteria for studying crack growth. This type of hold time pattern was used in the NASA study referred to above.
Progress has been made in forming superalloy metal compositions containing high volume percents of strengthening precipitates and in processing of these metals into parts for advanced turbine engines and jet aircraft engines. This metal processing technology has been developed to introduce such superalloys into gas turbines and jet engines because of the higher temperature capabilities of the alloys themselves and because the engines built with such alloys also have higher temperature capabilities and resulting higher efficiencies and thrust per unit weight of engine. While some studies such as the NASA studies described above have been made of a number of these alloys, not all of the alloys have been examined comprehensively with respect to fatigue cracking and with respect to resistance to fatigue cracking.
It has been determined that at low temperatures the fatigue crack propagation depends essentially entirely on the intensity at which stress is applied to components and parts of such structures in a cyclic fashion. As is partially explained in the background statement above, the crack growth rate at elevated temperatures cannot be determined simply as a function of the applied cyclic stress intensity .DELTA.K. Rather the fatigue frequency can also affect the propagation rate. The NASA study demonstrated that the slower the cyclic frequency, the faster the crack grows per unit cycle of applied stress. It has also been observed that faster crack propagation occurs when a hold time is applied during the fatigue cycle. Time-dependence is a term which is applied to such cracking behavior at elevated temperatures where the fatigue frequency and hold time are significant parameters.
It is known that some of the most demanding sets of properties for superalloys are those which are needed in connection with jet engine construction. Of the sets of properties which are needed, those which are needed for the moving parts of the engine are usually greater than those needed for static parts although the sets of needed properties are different for the different components of an engine.
Because some sets of properties have not been attainable in cast alloy materials, resort is sometimes had to the preparation of parts by powder metallurgy techniques. However, one of the limitations which attends the use of powder metallurgy techniques in preparing moving parts for jet engines is that of the purity of the powder. If the powder contains impurities such as a speck of ceramic or oxide, the place where that speck occurs in the moving part becomes a latent weak spot where a crack may initiate or a latent crack.
As alloy products for use in turbines and jet engines have developed, it has become apparent that different sets of properties are needed for parts which are employed in different parts of the engine or turbine. For jet engines, the material requirements of more advanced aircraft engines continue to become more strict as the performance requirements of the aircraft engines are increased. The different requirements are evidenced, for example, by the fact that many blade alloys display very good high temperature properties in the cast form. However, the direct conversion of cast blade alloys into disk alloys is very unlikely because blade alloys display inadequate strength at intermediate temperatures of about 700.degree. C. Further, the blade alloys have been found very difficult to forge and forging has been found desirable in the fabrication of blade from disk alloys. Moreover, the crack growth resistance of disk alloys has not been evaluated.
Accordingly, to achieve increased engine efficiency and greater performance, constant demands are made for improvements in the strength and temperature capability of disk alloys as a special group of alloys for use in aircraft engines. Now these capabilities must be coupled with low fatigue crack propagation rates and a low order of time-dependency of such rates.
What was sought in undertaking the work which led to the present invention was the development of a disk alloy which had superior strength at temperatures above 1200.degree. F. and which had a lower or minimum time dependence of fatigue crack propagation and moreover a high resistance to fatigue cracking. Further, what was sought, was high strength and long stress rupture life.
A main objective was to provide a composition which has high strength at temperatures above 1200.degree. F. together with a high resistance to time dependent fatigue crack propagation. One way in which this objective is achieved is through extending the cooling rate for preparation of the composition. One thing that is highly desirable in the composition of this invention is that it permits a broad range of variation in the cooling rate but still provides the desired combination of strength at high temperature together with resistance to time dependent fatigue crack propagation. In other words this is a unique alloy because it not only has high temperature strength coupled with time dependent fatigue crack propagation resistant properties but these properties are achieved and can be achieved over a broad range of cooling rates.
In addition the accomplishment of the high strength capability coupled with time dependent fatigue crack propagation resistance at this broad range of cooling rates does not detract from other properties of the alloy as the alloy itself does have a good combination of strength and rupture life. Also this exceptional combination of high temperature strength and other properties is achieved at cooling rates over any part of a broad range where such cooling is responsible for the resulting properties of the time dependent fatigue crack propagation resistant alloy. In other words there is no significant loss of higher temperature strength or rupture life properties over this whole cooling rate range. The alloy which is prepared according to the present invention not only has the good resistance to fatigue crack propagation but it has good high temperature strength at temperatures above 1200.degree. F. and good rupture life.